In recent years, for semiconductor devices, an increase in integration density and speed is significant from the viewpoint of coping with an increase in performance and a reduction in size and weight of electronic equipment. Packages having various structures have been proposed in order to realize a reduction in size and to avoid problems of increased calorific value and noise involved in the above tendency. In particular, a structure wherein a heat sink is previously mounted on a lead frame and a structure wherein a heat sink is incorporated at the time of resin molding in the step of assembling a semiconductor device have been proposed as one means for realizing a plastic molded package structure having good heat dissipation.
The most widely used package is a plastic package prepared by transfer molding. The construction of such a plastic package is shown in FIG. 1. The plastic package comprises a semiconductor chip 3, a die pad 7, a bonding wire 6, a lead frame 2, and a mold resin 5. In this plastic package, the semiconductor chip 3 is loaded on the die pad 7 and is electrically connected through the bonding wire 6 to each lead of the lead frame 2, followed by molding using a mold resin 5, such as an epoxy resin. Thereafter, the exterior of an outer lead 12 of the lead frame 2 is then plated to form an outer plating 14, and a bent portion 9 as shown in the drawing or the like is formed to complete a plastic package.
In the case of low power consumption and microcomputers and memories where there is no special requirement for electrical characteristics, this plastic package has a satisfactory function as an inexpensive, high reliable package. On the other hand, in the case of devices where the power consumption is high and strict requirement for electrical characteristics should be met, various improvements have been made in the plastic package, leading to the development and practical use of a plastic package having low heat resistance and excellent electrical characteristics.
In particular, regarding an improvement in heat dissipation of a plastic package having the above construction, a radiator 1 is provided so as to thermally come into contact with a lead frame or a die pad to improve the heat dissipation. In one case, the radiator is provided within a resin package 5, and in other case, it is provided so that part of the radiator is exposed to the outside of the package. Copper and aluminum materials having high thermal conductivity are used as the material for the radiator 1. A radiator using aluminum is anodized to form an anodic oxide film 8 from the viewpoints of improving the corrosion resistance and improving the decorative property and is black colored particularly from the viewpoint of improving heat dissipation and the like.
A dye is used for coloring of the anodized radiator. The anodic oxide film has micropores extending perpendicularly to the surface thereof, and deposition of an organic black dye onto the micropores permits the anodic oxide film to be black colored. The organic dye forms a chelate with a metallic ion centered, thereby forming a chelate dye. Further, replacement of part of the dye molecule with other ions, for example, an alkali metal such as sodium or potassium, or addition of a salt, such as ammonium acetate, as a pH buffer is performed to improve the water solubility, dyeing property and other properties of the dye.
However, an alkali metal ion having high mobility in the semiconductor is unfavorably eluted from the dyed anodized film. In some applications, this leads to a fear of the reliability of the semiconductor element being deteriorated. There is an ever-increasing tendency for an increase in speed and capacity of electronic circuits, rendering measures against the generation of heat important. For this reason, provision of a colored radiator which, as compared with the conventional radiator, has better heat dissipation and is less likely to cause elution of ion components has been desired in the art.
Further, in the prior art, a gap is likely to be created between the radiator 1 and the mold resin 5. The presence of this gap results in remarkably deteriorated heat dissipation. Further, heating involved in brazing at the time of mounting on a printed board is likely to create cracking in the mold resin. For this reason, high adhesion is required between the radiator 1 and the mold resin 5. In the prior art, however, no satisfactory adhesion is provided between the aluminum alloy radiator 1 and the mold resin 5, and, in addition, cracking is likely to be created in the resin, unavoidably causing a deterioration in heat dissipation.